1. Field of the Invention
The present invention relates to a MOSFET formed on an SOI (Silicon On Insulator) substrate, and in particular to a method of detecting SPICE (Simulation Program With Integrated Circuit Emphasis) parameters, performing a SPICE calculation, and performing device simulation for a partially depleted SOI MOSFET where the body is only partially depleted.
2. Related Art
An SOI MOSFET is a MOSFET constructed in a single silicon crystal formed on an insulator film called an SOI (Silicon On Insulator) substrate. In particular, in a partially depleted SOI MOSFET with a floating body effect, each component of the currents that flow into a body current and causes changes in the amount of body charge (such currents are collectively referred to as the “body current”) is extremely small, which means that the fluctuations in the body charge are also extremely small. This means that during circuit operation, capacitive coupling occurs between the body and each of the nodes, i.e., the gate, the source, the drain, and the back gate. It is also known that with methods where a MOSFET is operated by providing a terminal on the body and fixing the potential, capacitive coupling occurs in the same way when the body or body terminal resistance and circuit operating frequency are both sufficiently high.
In this way, in an SOI MOSFET where capacitive coupling occurs in the body, during actual circuit operation, the total body charge affects not only the body potential but also the effective drain current characteristics. Naturally, when the effective drain current characteristics are estimated by performing device simulation and a SPICE calculation, it is necessary to calculate the total body charge during circuit operation.
An article by A. Wei et al. in IEEE Trans. Electron Devices, vol. 45, pp. 430–438, 1998 discloses a method of calculating, through device simulation, the total body charge of an SOI MOSFET during circuit operation.
In SPICE calculations of an SOI MOSFET, it is normally thought necessary to prepare models that reproduce the body current, that is, a diode current, an impact ionization current, and a tunnel current resulting in gate induced drain leakage (GIDL), and the capacitance-voltage characteristics (or charge-voltage characteristics) of the body.
For example, kinks that appear in the SOI MOSFET static characteristics (the output characteristics and the sub-threshold characteristics) are understood as a phenomenon where the potential of the body increases due to the impact ionization current. In order to reproduce this kink phenomenon in simulations, models of the current-voltage characteristics for the body current and body capacitance-voltage characteristics have been implemented in the standard models that are presently available (the SOI version of BSIM3 called “BSIMPD” by UC Berkeley and UFSOI by U. Florida, etc.).
However, with regard to models of the current-voltage and capacitance-voltage characteristics for the body (substrate) that are not so important in conventional bulk-MOSFET techniques, the need to perform a procedure for estimating and calculating coefficients for specifying the details of these models in keeping with actual device characteristics presents a considerable burden.
Since the body current that is an origin of the fluctuation in the total body charge during actual circuit operation is extremely small, after sufficient repetition of switching operations, the total charge of the body converges on a state where the charge fluctuates slightly about a certain value. During circuit simulation, it is necessary to calculate the convergence value for the total body charge in advance and to set this calculated value as the initial value for simulation. In addition, since the absolute value of the body current is an order of magnitude smaller than that of the driving current, the calculation of the total body charge and moreover the potential need to be calculated with high accuracy such that there are inevitable compromises between the time taken for the calculation (calculation time) and the degree of reliability of the convergence.
When performing circuit analysis on a partially depleted SOI MOSFET, it is necessary to estimate and calculate coefficients that determine the details of models of the current-voltage and capacitance-voltage characteristics for the body in order to meet actual characteristics, and to perform a procedure for calculating in advance the convergence value of the total body charge during circuit operation. That is, the procedure for extracting the SPICE parameters and performing a SPICE calculation is complex. While the calculations of the total charge and moreover the potential of the body need to be performed with high accuracy, the calculation time and convergence unavoidably have to be prioritized, so a compromise is made to the calculation accuracy.
The present invention was conceived in view of the situation described above, and attempts to increase the efficiency of the procedures for SPICE parameter extraction, SPICE calculations, and device simulation for a partially depleted SOI MOSFET.